Through its major derivative, sulfuric acid, sulfur ranks as one of the most important elements used as an industrial raw material. It is of prime importance to every sector of the world""s industrial and fertilizer complexes. Sulfuric acid production is the major end use for sulfur, and consumption of sulfuric acid has been regarded as one of the best indexes of a nation""s industrial development. More Sulfuric acid is produced in the United States every year than any other chemical, nearly 48 million metric tons. The World production of sulfuric acid is over 150 million metric tons per year.
There is a wide range of industrial applications for sulfuric acid. Some examples of these include uses in: phosphorus and nitrogen fertilizers; petroleum refining; mineral leaching, i.e. copper, zinc, nickel and titanium extraction; industrial organic and inorganic chemical production; the processes for manufacturing paints and pigments; the iron, steel and non-ferrous metallurgy industry; the production of rayon and cellulose film; pulp and paper; and water treatment. Because of its desirable properties, sulfuric acid has retained its position as the most universally used mineral acid and the most produced and consumed inorganic chemical, by volume.
Sulfuric acid is typically produced via a catalyzed transformation of sulfur dioxide (SO2) into sulfur trioxide (SO3), followed by the reaction of SO3 with water to form sulfuric acid. Typically, the source of the SO2 is either by the direct burning of elemental sulfur or via base metal smelting (e.g. Copper, Zinc and Lead). Even though the process technologies for both metals based and direct sulfur burning to capture SO2 from release have been greatly improved, these process are still only capturing between 95% to 99% of the emissions. Old technologies based smelters where the emissions regulations are significantly less stringent, are located in remote locations mainly in South America, Southern Africa and China where they have largely been unaffected by mounting environmental pressures. However, even these remotely located smelters will be under increasing pressure in the future to reduce the quantity of noxious emissions. The retrofit would be capital-intensive. Accordingly, it would be advantageous to have a process for producing sulfuric acid which eliminates the hazards to the environment that are associated with the current production of sulfuric acid.
Additionally, operations such as metal leaching often have mines and plants located in remote areas and in countries lacking the infrastructure necessary to handle the extremely dangerous concentrated sulfuric acid. Most leaching operations use aqueous solutions of sulfuric acid that contain less than 20 grams per liter of sulfuric acid. With current technology, sulfuric acid is made in very concentrated form and then shipped to the point of use where it is diluted with water to produce the aqueous solutions used in most leaching operations. Accordingly, it would be advantageous to have a process which permits generation of aqueous sulfuric acid solutions very near the point of use and eliminates the hazards associated with the transport and handling of concentrated sulfuric acid.
As illustrated above, there is a long felt need for a more cost efficient and environmentally friendly process for producing sulfuric acid. The subject invention provides an alternative process for producing sulfuric acid that utilizes commercially available non-hazardous sulfur and/or sulfide ores and/or minerals that allow sufficient solid/liquid/gas transfer for commercial production. The process would be significantly lower in capital cost than either of the current processes and it reduces the environmental impact by nearly eliminating emissions.
The concept of producing sulfuric acid in small-scale submerged reactors utilizing biological means has been discussed in the literature by Cerruti et al., Bio-dissolution of Spent Nickel-Cadmium Batteries using Thiobacillus Ferroxidans, Journal of Biotechnology 62, 209-211 (1998); Curutchet et al., Combined Degradation of Covellite by Thiobacillus Thiooxidans and Thiobacillus Ferrooxidans, Biotechnol. Lett. 18, 1471-1476 (1996); Tichy et al., Possibilities for Using Biologically-Produced Sulphur for Cultivation of Thiobacilli with Respect to Bioleaching Processes, Bioresource Technology 48, 221-227 (1994); Tichy et al., Oxidation of Biologically-Produced Sulphur in a Continuous Mixed-Suspension Reactor, Wat. Res. Vol. 32, 701-719 (1998); Otero et al., Action of Thiobacillus Thiooxidans on Sulphur in the Presence of a Surfactant Agent and its Application in the Indirect Dissolution of Phosphorus, Process Biochemistry, Vol. 30, 747-750 (1995); and Brissette et al., Bacterial Leaching of Cadmium Sulphide, The Canadian Mining and Metallurgical (CIM) Bulletin for October, 1971, 85-88 (1971). However, in these studies the elemental sulfur used was described as powder, flower, crystalline or biological for small laboratory operations. Tichy (1994) indicated that acid production rates using elemental-sulfur flower are too low and industrial applications of this process are doubtful. Accordingly, it would be advantageous to have a process that could generate sulfuric acid through biological means at production rates that are suitable for industrial applications.
It is an object of this invention to provide a novel process and apparatus for the biological production of sulfuric acid (H2SO4).
Another object of this invention is to produce a low cost process and apparatus capable of producing sulfuric acid product at production rates suitable for industrial applications.
Another object of this invention is to provide a more environmentally friendly process for producing sulfuric acid than either direct burning of the elemental sulfur or base metal smelting.
Yet another object of this invention is to provide a process for producing sulfuric acid that can be safely and economically located at the point of use for the sulfuric acid; thus, eliminating transportation and handling concerns.
Other objects and advantages will be apparent from the detailed description of the appended claims.
The subject invention pertains to the biological production of sulfuric acid (H2SO4). In a preferred embodiment, the method of the subject invention involves using oxidizing bacteria or acidophilic fungi to oxidize elemental sulfur or pyrite to form sulfuric acid. In a specific embodiment, the sulfur is treated with water in the presence of sulfur oxidizing microbes. In a specific embodiment, the bioleaching can be performed using the oxidizing bacterium Thiobacillus Thiooxidans. 
According to one aspect of the invention a process for producing sulfuric acid is provided. The process comprises contacting an aqueous solution with a sulfur material in the form of a pile. The sulfur material is selected from the group consisting of elemental sulfur, sulfur-containing ores, sulfide-containing ores, sulfur-containing minerals, sulfide-containing minerals and combinations thereof. The pile additionally contains acidophilic microbes and, preferably, a packing material. The pile is aerated with an oxygen-containing gas and a liquid stream is withdrawn for the pile. A first portion of the liquid stream is returned to the pile for further contacting with the pile and a second portion of said liquid stream is taken as an acid product.
In accordance with another aspect of the invention an apparatus for producing sulfuric acid is provided. The apparatus comprises at least one reaction vessel having a base. The reaction vessel contains a bottom layer adjacent to the base. The bottom layer is comprised of a first packing material. A reactant layer is located above the bottom layer wherein the reactant layer contains a sulfur material selected from the group consisting of elemental sulfur, sulfur-containing ores, sulfide-containing ores, sulfur-containing minerals, sulfide-containing minerals and combinations thereof and an acidophilic microbe. The apparatus further comprises an aerator extending at least partially into the bottom layer. The aerator introduces an oxygen-containing gas into the bottom layer such that the oxygen-containing gas flows upward and through the reactant layer. An irrigation system extending over the reactant layer introduces an aqueous solution at or above the top of the reactant layer such that the aqueous solution flows down and through the reactant layer and into the bottom layer. During the aqueous solutions passage through the reactant layer a biological reaction takes place to produce sulfuric acid and, hence, the aqueous solution at the bottom layer is an acid solution containing sulfuric acid. A suitable means for withdrawing the acid solution from said reaction vessel is utilized and the withdrawn acid solution is split into a first and second portion. The first portion is introduced to the top of said reactant layer and the second portion is withdrawn as an acid solution product.
In the preferred embodiment, the process involves using acidophilic microbes to oxidize a variety of forms of elemental sulfur, sulfide-containing ores or minerals (i.e. pyrite and chalcopyrite), and sulfur-containing volcanic tuffs. The sulfur used in the process can be in a variety of forms. These forms may include prills, pastilles, slates, flowers, dusts, or sulfur coated onto a substrate. The pile of sulfur and/or the sulfide-containing ores or minerals can be contacted with the liquid and gas phases in a reaction vessel or a freestanding heap.
The aqueous solution, which can be distilled water, potable water, and/or an acidic or non-acidic industrial stream, is contacted with the top of solid material at either constant or intermittent rates. The primary requirement of the incoming liquid stream is that it does not contain any components that would be toxic or inhibitory to the microbial life. However, for a specific industrial application with a toxic or inhibitory component, a person skilled in the art can slowly adapt acidophilic microbes to tolerate the toxin. The oxygen-containing gas stream, preferably air, would be introduced into the pile or reactor at the appropriate rates to assure sufficient amount of oxygen for sulfuric acid production.
Also in accordance with this invention, the resulting concentration of the sulfuric acid of the subject invention can be upgraded in acid strength by a number of commercial techniques. These include, but not limited to, reverse osmosis, membrane separation, filtration, distillation, and cryogenics methodology. The waste stream from the upgraded methodology can be returned to the reaction vessel or freestanding pile keeping the entire system with zero discharge. Even at the shutdown of the plant, the remaining inoculated sulfur source can be valuable for a new plant startup. Makeup aqueous streams can be used to maintain the appropriate water balance.